Quantitative Precipitation Forecast
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FXUS04 KWBC 191056

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
656 AM EDT Mon Mar 19 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 19/1200 UTC thru Mar 22/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Eastern U.S...

A very dynamic southern stream trough will move through the
Tennessee Valley today, inducing a widespread precipitation event
from the Plains to the east coast - and some concentrated severe
weather per the Storm Prediction Center Day 1 Convective Outlook -
associated with a well defined frontal wave and the extensive
back-bent occluded front. The inherited WPC forecast served as a
good starting point. Areal average precipitation amounts will be
greater north of the warm front and along the occlusion, with the
aide of upglide over the frontal surface and deformation. Down
along the front and into the warm sector storms will be moving
quickly, with 70-80 knot westerly 500 mb flow. QPF will be less in
the warm sector, but we did attempt to be generous with coverage,
and carry at least tenth of an inch amounts down farther south
than some of the hi-res models owing to sufficient deep layer
height falls in a favorably unstable and uncapped environment.

WPC QPF was really a laborious manual effort given the level of
mesoscale influence expected today, but several pieces of guidance
were close to our expectations, especially the 00z NAM CONUS Nest
and GEM Regional. With the heavier local rain rates moving quickly
today, and less convection within the longer duration rainfall in
KS/MO/IL, there does not appear to be cause for more than a
Marginal Risk of excessive rainfall at any given location.

...Central and Northern Plains...

While low levels will feature dry ridging over the High Plains,
and northerly winds influenced in part by the mid Mississippi
Valley cyclone, there will be plenty of residual moisture in the
mid levels and cyclonic flow / digging northern stream features to
support light rain/snow from Kansas to North Dakota. With
precipitation forced primarily in the 700-500 mb layer, the SREF
6-hourly precip probabilities seemed to match well with the GFS
700-500 mb lift and saturation fields. We used these as a guide,
and came up with QPF resembling the 00z HREF mean.

Days 2/3...

...West Coast/Rockies/Great Basin...

Ahead of a strong mid level system approaching the West Coast
during Days 2 and 3, deep moisture associated with an atmospheric
river will affect much of CA, especially during Day 3. The deep
moisture and strengthening upslope flow over portions of central
and Southern CA could result in significant rainfall and the
potential for flash flooding, as well as mudslides and debris
flows in burn scar areas. While there is fairly good agreement
among models with respect to the overall synoptic setup, there is
still some spread concerning where the moisture becomes focused.
In an attempt to mitigate some of the differences (mainly between
the 00z ECMWF/GFS), the WPC QPF was based on a blend of these
models. weighted more heavily toward the 00z ECMWF (as the 00z GFS
continues to be too fast approaching the coast).

Day 2...
A closed mid level low located near 33N 140W at the start of Day 2
evolves into a positively tilted long wave trough as it moves
closer to the CA coast by the end of the period. Ahead of the long
wave trough, there is a multi model signal for deep moisture
associated with a well advertised atmospheric river (a feature
which the GFS has been showing for a few days now) to cross
southwest CA after 21/00z. Precipitable water values near 1.25
inches (which is between four and five standard deviations above
the mean) are ushered ashore by a 20 to 30 knot low level
southwest flow.

The flow become more orthogonal with time to the Transverse Ranges
in Southern CA, resulting in a better upslope component between
21/06z and 21/12z. Model soundings off the southwest CA coast
showed marginal instability in the moisture plume, with the best
synoptic scale lift expected late in the period. The ingredients
are apparently coming together for a significant rainfall event
for portions of Southern CA, which most 00z model solutions
showing some version of the above occurring, yielding between 1.00
and 1.50 inches of qpf along the southwest CA coast.

However, timing issues remain between the faster GFS and slower
ECMWF. Because of these timing differences, it is possible that
the best ingredients for a significant rainfall event occur after
21/12z, or the end of Day 2. The Slight Risk area in place on Day
3 has been moved forward into Day 2 with little in the way of
change (other than to account for timing differences).

Day 3...
The strong positively tilted long wave trough off the West Coast
approaches the Pacific Northwest late on Day 3. A strong mid level
closed low forms off the WA coast, and the low level southwest
flow ahead of the mid level and surface low (which tracks west of
the Pacific Northwest coast) transports 0.75 inch precipitable
water air over the western WA/western OR. The best upslope flow
occurs over the OR Cascades, where local 0.75 inch qpf amounts are
expected. Some of the moisture survives the trip over the terrain
into the Northern Rockies, where upslope flow produces local
0.25/0.50 inch qpf amounts over the Blue Mountains in OR, the
Sawtooth Range in ID, and the Grand Tetons of WY.

Further south, the low level southwest flow becomes focused on
central and Southern CA ahead of a cold front associated with the
low off the Pacific Northwest coast. The best lift associated with
pieces of short wave energy in the mid level flow occur over the
central CA coastal range into the Transverse Ranges in Southern CA
after 22/00z. The low level southwest flow increases to 30 to 40
knots, transporting 1.25/1.50 inch precipitable water air (which
is approaching five standard deviations above the mean) from the
central CA coast to the mountains north of LA. Model soundings in
the deep moisture plume showed MUCAPE (in the 100/250 j/kg range)
reaching these areas, mainly after 22/00z. The low level flow
becomes orthogonal to the terrain, resulting in strong upslope
component, putting the ingredients in place for excessive rainfall
over a large area.

As mentioned earlier, there is fairly good model agreement
concerning the synoptic scale features, but there are important
differences in the focusing of the low level flow and deep
moisture. The 00z ECMWF continues that models solution of placing
3.00/4.00 inch qpf amounts closer to KSBP during Day 3, while the
00z GFS is faster and places between 5.00 and 6.00 inches of qpf
over the coast from KSBA and KSMO. Not surprisingly, the ensemble
means support their operational runs, so the WPC QPF attempts to
show these possible solutions. With this in mind, and after
collaborating with WFO LOX, a Moderate Risk was placed over these
areas for Day 3.

The low level flow also supports heavy rainfall over the central
CA coast range, as well as the Sierra Nevada range. Since the low
level flow is orthogonal to these ranges, a Slight Risk was placed
in these areas for Day 3. Across the Sierra Nevada range, there
was a strong model signal for 3.00 to 4.00 inches of qpf, and snow
levels are probably not going to be low enough to preclude the
potential flooding due to the upslope flow.

...OH Valley/Mid Atlantic/Northeast...

Short wave energy tracking out of the Northern Plains and short
wave energy in the southern stream crossing the Lower MS Valley
aid in carving out a long wave trough extending from the OH Valley
into the Mid Atlantic states during Day 2, then takes on a
negative tilt over the Northeast during Day 3. The mid level lift
supports the development of a pair of surface lows impacting the
northern Mid Atlantic and Northeast. There is still a fair amount
of model spread concerning the evolution of the mid level systems,
and the track of the subsequent surface lows. In attempt to
mitigate some of the differences, the WPC QPF was based primarily
on a blend of the 00z GFS/12z EMCWF.

Day 2...
The first short wave in the northern stream tracks toward the Mid
Atlantic coast early on Day 2, which supports deepening of a
surface wave off the southern Delmarva coast. Banding associated
with the surface low looks as though it will remain along the
DE/southern NJ coast, where an area of 0.50 to 1.00 inches of qpf
was placed. Further west, a weakening surface low over OH/WV and
its frontal boundary serves as a focus for moisture ahead of the
short wave crossing the region, and a large area of 0.25 to 0.50
inches of qpf was placed over
OH/KY/eastern TN across much of VA/MD. There is some still some
model spread with respect to the track of the surface low and
extent of the moisture over the northern Mid Atlantic, so there
could still be come changes with subsequent model runs.

Day 3...
As the southern stream short wave reaches the Mid Atlantic states
late on Day 2, it helps develop a negatively tilted long wave
trough extending from the OH Valley to the Mid Atlantic coast at
the start of Day 3. A second surface low forms off the NC coast
early in the period, then tracks northeast with a closing mid
level system as to moves near the New England coast. The 00z
GFS/00z CMC follow this track, while the 00z UKMET/00z ECMWF are
closer to the coast (with the 00z ECMWF furthest west with the
surface and its it associated frontogenetic bands). Since there
has been some model volatility with the ECMWF solution over the
last three runs, the 12z ECMWF was used instead to augment the 00z
GFS to produce the WPC QPF.

The highest QPF amounts were placed over eastern MA, where there
was a better model consensus for 0.50 to 1.00 inches of QPF.
Further southwest across southern New England into the NYC metro
and NJ/PA, the model differences are much more stark, with the
westernmost solutions producing much higher qpf amounts than shown
by the WPC QPF. Because of the model shifts, this has become a
lower than average confidence forecast. Changes in the western
edge of the more significant QPF could occur with subsequent model


Deep moisture and strong instability streaming ahead of a cold
front crossing FL during day 2 will feed convection that produces
locally heavy rainfall amounts. There was generally good model
agreement with respect to the timing of the front, so the WPC QPF
was based mainly on a blend of the 00z ECMWF/GFS.

Short wave energy in the southern stream drops into the bottom of
the developing long wave trough over the Southeast states during
Day 2. The mid level trough provides ample synoptic scale lift
over a cold front crossing northern and central FL. Deep moisture
(with precipitable water values between 1.50/1.75 inches) and
moderate strong instability will supply developing convection
along and ahead of the front, and the synoptic scale lift will
help organize linear convection crossing the region between 21/18z
and 22/06z. There was a multi model signal for local 1.00 inch qpf
amounts over north central FL, before the expected line of
convection accelerates eastward. Because of the speed of the
convection, flash flooding is not expected with the storms.


Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml


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